CAP4730: Computational Structures in Computer Graphics

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CAP4730 Computational Structures in Computer
Graphics
3D Concepts
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Outline

• Basic Idea of 3D
• Projections
• What are some things we didnt have to worry
  about before?
• What are some new things we can do?

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Right Handed Coordinate System
Y
Y
Z
X
X
Z
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Viewing a 3D world
Y
We have a model in this world and would like to
view it from a new position. Well call this
new position the camera or eyepoint. Our job is
to figure out what the model looks like on the
display plane.
X
Z
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Parallel Projection
Y
Z
X
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Perspective Projection
Y
Z
X
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What are some new things to think about?
Hidden Surface Removal Visibility Depth Cueing
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How to make a 2D image appear as 3D!

• Output is typically 2D Images
• Yet we want to show a 3D world!
• How can we do this?
• We can include cues in the image that give our
  brain 3D information about the scene
• These cues are visual depth cues

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Visual Depth Cues

• Monoscopic Depth Cues (single 2D image)
• Stereoscopic Depth Cues (two 2D images)
• Motion Depth Cues (series of 2D images)
• Physiological Depth Cues (body cues)

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Monoscopic Depth Cues

• Interposition
• An object that occludes another is closer
• Shading
• Shape info. Shadows are included here
• Size
• Usually, the larger object is closer
• Linear Perspective
• parallel lines converge at a single point
• Surface Texture Gradient
• more detail for closer objects
• Height in the visual field
• Higher the object is (vertically), the further it
  is
• Atmospheric effects
• further away objects are blurrier
• Brightness
• further away objects are dimmer

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Stereoscopic Display Issues

• Stereopsis
• Stereoscopic Display Technology
• Computing Stereoscopic Images
• Stereoscopic Display and HTDs.
• Works for objects lt 5m. Why?

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Stereopsis
The result of the two slightly different views of
the external world that our laterally-displaced
eyes receive.
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Time-parallel stereoscopic images

• Image quality may also be affected by
• Right and left-eye images do not match in color,
  size, vertical alignment.
• Distortion caused by the optical system
• Resolution
• HMDs interocular settings
• Computational model does not match viewing
  geometry.

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Disparity

• If an object is closer than the fixation point,
  the retinal disparity will be a negative value.
  This is known as crossed disparity because the
  two eyes must cross to fixate the closer object.
• If an object is farther than the fixation point,
  the retinal disparity will be a positive value.
  This is known as uncrossed disparity because the
  two eyes must uncross to fixate the farther
  object.
• An object located at the fixation point or whose
  image falls on corresponding points in the two
  retinae has a zero disparity.

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Convergence Angles
f1

• ?acbd 180
• ?cd 180
• ?-? a(-b) ?1?2 Retinal Disparity

a
D1
f2
b
D2
a
b
c
d
?2
?1
i
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Stereoscopic Display

• Stereoscopic images are easy to do badly, hard to
  do well, and impossible to do correctly.

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Stereoscopic Displays

• Stereoscopic display systems create a
  three-dimensional image (versus a perspective
  image) by presenting each eye with a slightly
  different view of a scene.
• Time-parallel
• Time-multiplexed

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Time Parallel Stereoscopic Display

• Single Screen
• Two different images projected on the same screen
• Images are polarized at right angles to each
  other.
• User wears polarized glasses (passive glasses).

• Two Screens
• Each eye sees a different screen
• Optical system directs each eye to the correct
  view.
• HMD stereo is done this way.

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Passive Polarized Projection Issues

• Linear Polarization
• Ghosting increases when you tilt head
• Reduces brightness of image by about ½
• Potential Problems with Multiple Screens (next
  slide)
• Circular Polarization
• Reduces ghosting but also reduces brightness and
  crispness of image even more

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Problem with Linear Polarization

• With linear polarization, the separation of the
  left and right eye images is dependent on the
  orientation of the glasses with respect to the
  projected image.
• The floor image cannot be aligned with both the
  side screens and the front screens at the same
  time.

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Time Multiplexed Display

• Left and right-eye views of an image are computed
  and alternately displayed on the screen.
• A shuttering system occludes the right eye when
  the left-eye image is being displayed and
  occludes the left-eye when the right-eye image is
  being displayed.

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Stereographics Shutter Glasses
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Motion Depth Cues

• Parallax created by relative head position and
  object being viewed.
• Objects nearer to the eye move a greater distance

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Pulfrich Effect

• Neat trick
• Different levels of illumination require
  additional time (your frame rates differ base of
  amount of light)
• What if we darken one image, and brighten
  another?
• http//dogfeathers.com/java/pulfrich.html
• www.cise.ufl.edu/lok/videos/pulfrich.avi

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Physiological Depth Cues

• Accommodation focusing adjustment made by the
  eye to change the shape of the lens. (up to 3 m)
• Convergence movement of the eyes to bring in
  the an object into the same location on the
  retina of each eye.

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Summary

• Monoscopic Interposition is strongest.
• Stereopsis is very strong.
• Relative Motion is also very strong (or
  stronger).
• Physiological is weakest (we dont even use them
  in VR!)
• Add as needed
• ex. shadows and cartoons

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What are some new things we can do?

• Lighting and Shading!
• Texturing!
• Stereo
• Surfaces
• Normals
• Materials

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